Power of Java MemoryMapped File

Power of Java MemoryMapped File

In JDK 1.4 interesting feature of Memory mapped file was added to java, which allow to map any file to OS memory for efficient reading. Memory mapped file can be used to developed  IPC type of solution. This article is experiment with memory mapped file to create IPC.

Some details about Memory Mapped File, definition from WIKI

A memory-mapped file is a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on-disk, but can also be a device, shared memory object, or other resource that the operating system can reference through a file descriptor. Once present, this correlation between the file and the memory space permits applications to treat the mapped portion as if it were primary memory.

Sample Program

There are two java program one is writer and other is reader. Writer is producer and tries to write to Memory Mapped file , reader is consumer and it reads message from memory mapped file. This is just a sample program to show to idea, it does't handle many edge case but good enough to build something on top of memory mapped file.

MemoryMapWriter

import java.io.File;

import java.io.FileNotFoundException;

import java.io.IOException;

import java.io.RandomAccessFile;

import java.nio.MappedByteBuffer;

import java.nio.channels.FileChannel;

public class MemoryMapWriter {

public static void main(String[] args) throws FileNotFoundException, IOException, InterruptedException {

File f = new File("c:/tmp/mapped.txt");

f.delete();

FileChannel fc = new RandomAccessFile(f, "rw").getChannel();

long bufferSize=8*1000;

MappedByteBuffer mem =fc.map(FileChannel.MapMode.READ_WRITE, 0, bufferSize);

int start = 0;

long counter=1;

long HUNDREDK=100000;

long startT = System.currentTimeMillis();

long noOfMessage = HUNDREDK * 10 * 10;

for(;;)

{

if(!mem.hasRemaining())

{

start+=mem.position();

mem =fc.map(FileChannel.MapMode.READ_WRITE, start, bufferSize);

}

mem.putLong(counter);

counter++;

if(counter > noOfMessage )

break;

}

long endT = System.currentTimeMillis();

long tot = endT - startT;

System.out.println(String.format("No Of Message %s , Time(ms) %s ",noOfMessage, tot)) ;

}

}